1. Field of Invention
In general, the present invention relates to homodyne detectors, in which a received carrier signal is mixed with locally generated in-phase and quadrature phase signals each having the same frequency as the carrier signal, in order to produce an in-phase (I) and quadrature-phase (Q) output signal components carrying a desired baseband signal. More particularly, the present invention relates to a method and apparatus for recovering the desired baseband signals from the in-phase and quadrature-phase signal components, while correcting for phase error present between these input signal components.
2. Brief Description of the Prior Art
Homodyne receivers for use in cable television converters are generally known in the art. U.S. Pat. No. 4,979,230 to applicant, incorporated herein by reference, discloses one such receiver. For purposes of illustration, this receiver is shown in FIG. 1.
As illustrated in FIG. 1, receiver 1 comprises a number of components. In particular, a filter and amplifier 2 are provided for filtering and amplifying an input signal 3 such as a vestigial side-band (VSB) television picture signal. The filtered and amplified input signal is then mixed at mixer 4 with the output of a tunable oscillator 5. This mixing process translates the frequency spectrum of the VSB input signal to a frequency band above that of the incoming signal. In this way, tuning over a range of 500 MHz (e.g. 50-550 MHz) in cable television applications is greatly simplified using a simple local oscillator. An intermediate frequency (IF) filter 6 then receives the upconverted spectrum from mixer 5 and attenuates most of the undesired spectrum, passing a single channel signal to homodyne mixer circuit 7.
At homodyne mixing circuit 7, a signal splitter 8 splits the desired upconverted channel signal into two portions. The first signal portion 9 is applied as input to mixer 10 and the second signal portion 11 is applied as input to mixer 12. A local oscillator 13 generates a fixed local carrier signal having a frequency which is offset to approximately the middle of the desired channel band, which in cable television applications is selected to be 2.15 MHz. This condition avoids distortion due to adjacent channel interference upon homodyne mixing. The local carrier signal from local oscillator 13 is provided to mixer 10 so as to mix with the first signal portion 10 and produce an in-phase output (I) signal component. Also, the local carrier signal output from local oscillator 13 is phase shifted by 90.degree. and provided to mixer 12 so as to mix with second signal portion 11 and produce a quadrature-phase output (Q) signal component. Notably, both of these signal components carrying the desired baseband signal, are then provided as input to a signal processor 14 for recovery of the original video signal. With such homodyne frequency conversion, a very low intermediate frequency (i.e. 2.15 MHz) is provided, permitting the use of conventional digital technology in subsequent signal processing in processor 14. The recovered baseband video signal is then provided as output to a television receiver or the like.
As illustrated, the I and Q signal components can be each provided to an analog-to-digital (A/D) converter 15 and 16, respectively, to convert these signals into a digital format. The digitized signals from A/D converters 15 and 16 can be expressed as Vsin(.beta.) and Vcos(.beta.), respectively, where .beta.=(.OMEGA..sub.LO -.OMEGA..sub.c)t, .OMEGA..sub.LO is the frequency of the local oscillator 13, and .OMEGA..sub.c is the intermediate frequency. The I signal component, Vsin(.beta.), is passed through a low-pass filter 17 which is tuned to .beta.=2.15 MHz in cable television applications. The output of low-pass filter 17 is coupled to the input of a digital phase lock loop circuit 18 that precisely locks onto the incoming 2.15 MHz signal and generates 0.degree. and 90.degree. components (i.e. local in-phase and quadrature-phase components) corresponding to Sin.beta. and Cos(.beta.). The Sin(.beta.) signal component is multiplied in a first mixer 19 with the Vsin(.beta.) signal component to produce a first product signal VSin.sup.2 (.beta.). Similarly, the Cos(.beta.) component is multiplied in a second mixer 20 with the VCos(.beta.) signal component to produce a second product signal VCos.sup.2 (.beta.). Then, when signal components VSin.sup.2 (.beta.) and VCos.sup.2 (.beta.) are added together in summing circuit 21, the resultant output on terminal 22 is the recovered desired baseband signal V.
While the above-described signal processor is capable of recovering baseband signals, it is, however, highly sensitive to phase error present between the in-phase and quadrature-phase signal components produced in the system. Consequently, such phase error typically results in unacceptable distortion of the recovered baseband signal.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for recovering a desired baseband signal from a detected signal having in-phase and quadrature-phase signal components, while automatically correcting for phase error present between these signal components.
Another object of the present invention is to provide such a method and apparatus, in which the phase error between the in-phase and quadrature-phase signal components is automatically corrected by introducing a predetermined amount of phase correction in one of the in-phase and quadrature-phase signal components.
Another object of the present invention is to provide such a method and apparatus, in which the selected amount of introduced phase correction is derived on the basis of sensed signals processed in real-time according to a trigonometric relation involving the sensed signals.
Another object of the present invention is to provide such apparatus in the form of a quadrature demodulation circuit having automatic phase-error correction capabilities.
Yet another object of the present invention is to provide such a quadrature demodulation circuit which is particularly adapted for digital implementation in a cable television converter, and capable of recovering desired baseband video signals with minimum distortion.
These and other objects of the present invention will become apparent hereinafter.